1. Field
The following description relates to an image forming apparatus and a controlling method thereof, and more particularly, to an image forming apparatus capable of adjusting a line sync signal without changing a motor speed to perform an OPC AC compensation, and a controlling method thereof.
2. Description of the Related Art
In general, an image forming apparatus such as a printer, copying machine, multifunction copier, and facsimile and the like that use an electronic photographing method is provided with an optic injector. The image forming apparatus forms an electrostatic latent image on a surface of a photosensitive medium using an optic beam output from the optic injector, and then transcribes the image to a piece of paper and performs an operation of printing a desired image.
In the past, an LSU (Laser Scanning Unit) was mostly used as an image forming apparatus that performs the role of an optic injector. As illustrated in FIG. 1, an LSU system used a method of adjusting an optic path reflected from a rotating polygon motor to form an electrostatic latent image on a desired point of the OPC photosensitive medium. Recently, image forming apparatuses that use an LPH (LED Print Head) are being developed. As illustrated in FIG. 2, an LPH uses a method of adjacently arranging a plurality of LED arrays and exposing in pixel units.
An LSU color image forming system of a tandem method is generally configured as in FIG. 3. Print data output from a main controller goes via the LSU and forms an image on each corresponding OPC. Herein, an error in a driving system of the OPC causes an AC component corresponding to an OPC cycle. When an image pattern of equal gaps is printed as in FIG. 4, and the gap of the actual printed pattern is measured through a sensor, the error in a location as in FIG. 5 represents the AC component.
In an LSU system, a sync signal in a main injection direction that forms an image on the OPC is generated using a BD (Beam Detect) signal that occurs while a polygon mirror rotates. This is configured to operate completely separate from the driving system that drives the OPC. Conventional methods for OPC AC compensation include a method of eliminating the AC component by controlling a rotation speed of the OPC and a method of reducing a registration error caused by the AC component by matching a mechanical phase so that the AC component of each color coincides to one another. These are methods of controlling the OPC motor. When intending to perform an AC compensation as aforementioned, a motor must be controllable per color. For this purpose, there needs to be provided a motor for each OPC, and thus there exists a problem of increasing manufacturing cost. Furthermore, the method of matching the mechanical phase so that the AC component of each color coincides to one another leads to a problem of deteriorating manufacturing productivity.
In the LSU system, there needs to be just one data line to be input to a laser diode. However, in the case of an LPH system, an array configuration matched to the size area of a page drives numerous devices at the same time, and thus more data lines are needed compared to the LSU system.
Furthermore, in the case of a color image forming apparatus, there needs to be 4 times more data lines than a black and white image forming apparatus.
For such reasons, it is not suitable for an SoC (System on Chip) or main controller of an image forming apparatus to include an LPH controller. Accordingly, in an image forming system using LPH, the main controller and LPH controller are usually separated from each other. In generally, the LPH controller is adjacent to an LPH module, and is connected to the main controller using a relatively long cable.
In the case of applying the LPH technology to the main controller and SoC developed for a conventional LSU, if not using a video interface for LSU use, a separate parallel interface must be used for transmission of printing data. In order to support multi-bit data for expressing multi tones for high resolution, the amount of data to be transmitted must be twice the amount in the case of 2 bit, and thus the line width of transmission data must be increased or the transmission rate of the data must be increased to twice the rate. Furthermore, at the receiving side, a frequency of a data clock for print data latching must be increased to twice the frequency. In the case of an image forming apparatus for LPH use, in order to prevent error due to a relatively long transmission distance between the main controller and LPH controller, a differential signal such as an LVDS is used. Accordingly, when using the method of increasing the line width of the transmission data for multi-bit transmission, the line width increases to twice the width. In order to increase the transmission rate of the transmission data without increasing the line width, the data clock frequency must be increased to twice the frequency, and thus there occurs a problem of limitations in the high speed high resolution system.